It surely sounds like either:
1) The video card
2) Incorrect XP drivers.
By the way, I'm also trying to install XP Pro x64 onto my Dell StudioXPS X35T/9000.
So far, my only limitations have been
1) No floppy drive in computer for F6 drivers
2) No XP 64-bit (or any) drivers for the X35T/9000 on Dell Support site.
Where did you get your XPPro x64 drivers? I surely would appreciate link(s) to the driver-set...
Donald L McDaniel